A digital circuit interrupter capable of complete implementation by means of very large scale integrated circuit techniques is described within U.S. patent application Ser. No. 631,708 filed July 17, 1984 in the name of John J. Dougherty. The circuit provides digital implementation of overcurrent determination and time delay for interrupting current through a circuit upon comparison with predetermined current values. The circuit employs a peak detecting A/D converter wherein the composite sensed circuit current is compared with the output of a D/A converter and a fixed clock pulse is compared to the output of this comparison. An I.sup.2 t digital value controlled by the peak value of the composite current is used as a measure of the current through the protected circuit.
When peak current is sensed within a protected circuit in industrial applications, there have been occurrences of false tripping caused by distorted current waveforms where 5th and 7th harmonic components can add to the peak value causing the peak value to exceed predetermined long time and short time pick-up values resulting in so-called "nuisance tripping" with accompanying loss in productivity. The electrical circuit must be independently tested to insure that no fault current actually exists before the circuit breakers can be reclosed and the industrial equipment can be made operational. This condition is more prevalent in electrical transmission and distribution systems employing power factor correction by the addition of large capacitor banks which tend to distort the sinusoidal waveform components. When highly inductive loads are employed, such as large induction furnaces, the sensed current waveform becomes harmonically distorted such that the 7th harmonic component adds to the peak current value causing an apparent overcurrent condition although the actual RMS current value is well below accepted limits. It has been determined that the current waveform distortion is not a problem when the RMS value of current is sensed, rather than the peak value as is now the custom in most industrial power circuits. RMS detection also provides a more realistic evaluation of the actual heating effect upon the power buses which are protected by the circuit interruption equipment within the protected circuit.
This invention therefore proposes a circuit whereby RMS calculation is made on a continuing basis as a measure of the composite current flowing through the protected circuit.